Off-line converter with digital control

ABSTRACT

A regulation circuit for use in DC to DC converter is disclosed. One such regulation circuit includes a feedback circuit to be coupled an energy transfer element output to receive a feedback signal to indicate whether the energy transfer element output is above or below a threshold level. A control circuit is included that is to be coupled to a switch that is coupled to an energy transfer element input. The control circuit is further coupled to the feedback circuit to generate a drive signal to control the switch to regulate the energy transfer element output to approximately the threshold level. The control circuit is to maintain an on-time of a current cycle of the drive signal and not allow an on-time of a next cycle of the drive signal in response to the energy transfer element output rising above the threshold level.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S.application Ser. No. 11/824,425, filed Jun. 28, 2007, now pending, whichis a continuation of U.S. application Ser. No. 11/389,184, filed Mar.24, 2006, now U.S. Pat. No. 7,248,029 B2, which is a continuation ofU.S. application Ser. No. 11/066,026, filed Feb. 25, 2005, now U.S. Pat.No. 7,038,439 B2, which is a continuation of U.S. application Ser. No.10/805,661, filed Mar. 18, 2004, now U.S. Pat. No. 6,876,181 B1, whichis a continuation of U.S. application Ser. No. 10/438,207, filed May 13,2003, now U.S. Pat. No. 6,747,444 B2, which is a continuation of U.S.application Ser. No. 10/092,705, filed Mar. 6, 2002, now U.S. Pat. No.6,608,471 B2, which is continuation of U.S. application Ser. No.09/927,273, filed Aug. 10, 2001, now U.S. Pat. No. 6,414,471 B1, whichis a continuation of U.S. application Ser. No. 09/630,477, filed Aug. 2,2000, now U.S. Pat. No. 6,297,623 B1, which is a divisional of U.S.application Ser. No. 09/032,520, filed Feb. 27, 1998, now U.S. Pat. No.6,226,190 B1.

BACKGROUND

1. Field of the Invention

The present inventions pertain to the field of power supplies, and amongother things to the regulation of power supplies.

2. Background

Accurate regulation of power supplies is important in many areas. Forinstance in sensitive electronic devices such as computers andtelevisions maintaining a constant power supply is important for theoperation of the computer or television. Additionally, the advantages ofaccurate power supply regulation include reduced overall powerconsumption and reduced damage to equipment by preventing voltage spikesduring start up and operation.

Power supplies are regulated by keeping either a current or voltagedelivered to a load within a specified range. A power supply is deemedto be in regulation if the load current or voltage is within thespecified range and is deemed to be out of regulation if the loadcurrent or voltage is outside the specified range.

Problems associated with out of regulation conditions include damage tothe load, improper load functioning, and the consumption of power whenno power is necessary to operate the load. Therefore, power suppliesthat regulate output power provided to the load are desired.

A known regulated power supply is depicted in FIG. 1. The regulatedpower supply of FIG. 1 includes an EMI filter 10 that receives an ACmains voltage. The output of the EMI filter 10 is coupled to rectifier15 that rectifies the AC mains voltage and then provides the rectifiedvoltage to capacitor 20. Capacitor 20 provides a substantially DCvoltage to a primary winding 25 of transformer 30.

A monolithic power supply control chip 40 includes a MOSFET 45 that iscontrolled by pulse width modulator 50. When MOSFET 45 is conducting,primary winding 25 has current flowing through it allowing transformer30 to store energy. When MOSFET 45 is not conducting, the energy storedin the transformer 30 induces a voltage across the secondary winding 55which is transferred to a load 60 connected at output terminals 65. Acapacitor 70 is coupled to secondary winding 55 in order to maintain thevoltage that is being supplied to the load 60 when MOSFET 45 is on.

A feedback circuit 75 is coupled to the load 60. The feedback circuit 75includes a resistor 80, zener diode 85 and an optocoupler 90. A biaswinding 95 is magnetically coupled to primary winding 25 and is used tosupply power to the output of the optocoupler 90. When the voltage atload 60 is above combination of the reverse bias voltage of zener diode85 and the forward voltage drop of light emitting diode 100, a currentis generated in the phototransistor 105 by light emitting diode 100. Thephototransistor 105 current flows from the bias winding 95 to thecontrol terminal 110 of monolithic power supply control chip 40. Thecurrent provided to the control terminal 110 of monolithic power supplycontrol chip 40 controls the duty cycle of MOSFET 45. When the controlterminal 110 current increases the duty cycle of MOSFET 45 decreases andthe amount of current through primary winding 25 decreases. Therefore,the power provided to the load 60 decreases. As the power supplied tothe load 60 decreases, the load voltage decreases which in turn reducesthe optocoupler 90 current increasing the duty cycle of MOSFET 45. Thus,the output voltage is regulated at a voltage equal to zener 85 reversebreakdown voltage plus the forward drop of LED 100 in an analog closedloop. Resistor 80 controls the gain of the analog loop.

It should be noted that pulse width modulator 50 is switching at someduty cycle to provide power to the feedback circuit 75 even when thereis no load connected to the output terminals 65. This will cause powerconsumption from switching losses occurring at the operating frequencyof the MOSFET 45.

The regulated power supply of FIG. 1 is able to maintain the voltage atthe load at a reasonably constant level, while reducing voltagetransients due to load and line variations.

However, the addition of a feedback winding and pulse width modulationcontroller makes application of the regulated power supply of FIG. 1expensive for many power suppliers operating at low powers, especiallythose below five (5) watts. Additionally, the use of analog pulse widthmodulation feedback control requires compensation circuitry to stabilizethe circuit and to prevent oscillations. The compensation circuit limitsthe bandwidth of the control loop to one (1) or two (2) kilohertz. ThePulse Width Modulated feedback circuit while effective at regulating thevoltage still has time periods when the voltage is above and below thedesired level, because of the limited bandwidth of the feedback loopwhich is in the range of one (1) or two (2) kilohertz even though theswitching frequency of the MOSFET 45 may be as high as one hundred (100)kilohertz.

It is therefore desired to create a power supply that is cost effectivefor low power solutions.

It is further desired to create a power supply that utilizes the minimumamount of components possible.

It is additionally desired to create a power supply that can respondquickly to load transients without losing output regulation.

SUMMARY OF THE INVENTION

A presently preferred DC to DC converter comprises an energy storageelement that receives a first power level and that provides a secondpower level, a feedback circuit coupled to the energy storage element,and a regulator circuit coupled to the feedback circuit and to theenergy storage element. When a feedback signal is above a threshold theregulator circuit is disabled and when the feedback signal is below saidthreshold level the regulator circuit is enabled.

In another embodiment a power supply comprises a transforming elementthat transfers energy and is coupled to receive a first power level anda regulator circuit coupled to the transforming element. The regulatorcircuit controlling input of the first power level to the transformingelement. When an output voltage or current of the transforming elementis above a threshold level the regulator circuit is disabled and whenoutput voltage or current of the transforming element is below athreshold level the regulator circuit operates.

In yet another embodiment a regulator circuit comprises a feedbackinput, a switch operating when a control signal is received at itscontrol terminal, an oscillator that provides a duty cycle signalcomprising a high state and a low state. The control signal is providedwhen no feedback signal is provided and the duty cycle signal is in saidhigh state.

In a further embodiment a power supply comprises an energy storageelement coupled to receive a first power level and a regulation circuitcoupled between the energy storage element and a source of the firstpower level. The regulation circuit prevents the energy storage elementfrom receiving the first power level when a current or voltage at theinput of the energy storage element is at or above a predeterminedthreshold level.

In an additional embodiment a power supply comprises a transformingelement coupled to receive a first power level and a regulation circuitcoupled between the transforming element and a source of the first powerlevel. The regulation circuit prevents the transforming element fromreceiving the first power level when a current or voltage at the inputof the transforming element is at or above a predetermined thresholdlevel.

It is an object of an aspect of the present inventions to create a powersupply that is accurately regulated with a minimum amount of time spentout of regulation.

It is another object of an aspect of the present inventions to create apower supply that is cost effective for low power solutions.

It is a further object of the present inventions to create a powersupply that utilizes the minimum amount of components possible.

It is also an object of the present inventions to create a power supplythat is low cost.

This and other objects and aspects of the present inventions are taught,depicted and described in the drawings and the description of theinvention contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a known regulated power supply.

FIG. 2 is a presently preferred regulated DC to DC power supplyaccording the present inventions.

FIG. 3 is a presently preferred power supply according to the presentinventions.

FIG. 4 is an alternate presently preferred power supply according to thepresent inventions.

FIG. 5 is a diagram of the presently preferred regulator circuit switchcurrent presently preferred power supplies of FIGS. 2, 3 or 4 accordingto the present inventions.

FIG. 6 is a functional block diagram of a presently preferred powersupply regulation circuit according to the present inventions.

FIG. 7 is a block diagram of a presently preferred bypass voltageregulation circuit according to the present inventions.

FIG. 8 is a diagram of the presently preferred bypass terminal voltageand maximum duty cycle signal according to the present inventions.

FIG. 9 is a block diagram of a presently preferred circuit allowing forincreasing the clock frequency of the oscillator according to thepresent inventions.

FIG. 10 is a block diagram of a presently preferred circuit allowing forstoppage of the oscillator according to the present inventions.

FIG. 11 is a diagram of the presently preferred enable signal and thesaw tooth waveform according to the present inventions.

FIG. 12 is a diagram of the signals generated in a presently preferredmode of operation within the presently preferred power supply regulationcircuit of FIG. 7 according the present inventions.

FIG. 13 is a diagram of the signals generated in a presently preferredmode of operation within the presently preferred power supply regulationcircuit of FIG. 9 according the present inventions.

FIG. 14 is a diagram of the signals generated in an alternate preferredmode of operation within the presently preferred power supply regulationcircuit of FIG. 10 according the present inventions.

DETAILED DESCRIPTION

Referring to FIG. 2, a DC to DC converter 200 receives a first DCvoltage 210 having a first magnitude. The first DC voltage 210 isconverted to a second DC voltage 220 that has a second magnitude byenergy storage element 205. Although, the presently preferred DC to DCconverter utilizes an energy storage element, other elements may be usedby the present invention without departing from the scope and spirit ofthe present invention. For instance a transforming element, may be usedas well. The second DC voltage 220 is provided to a load 230 to supplypower to the load 230. It is presently preferred that the second voltagelevel is below the first voltage level and that DC to DC converter 200is a step down converter. In operation the presently preferredregulation circuit 240 operates at fixed frequency, allowing current tobe provided into the energy storage element input 250 for a same timeperiod in-each cycle of the operating frequency. The output of feedbackcircuit 260 is utilized to enable or disable operation of the regulationcircuit 240. The magnitude of second DC voltage 220 will vary dependingon the ratio of the enable time to the disable time, i.e. the larger theratio the greater the magnitude of second DC voltage 220.

To maintain second DC voltage 220 at a regulated level feedback circuit260 is coupled to the positive terminal 270 of the load 230. A presentlypreferred feedback circuit 260 includes an optocoupler 280 and a zenerdiode 290. Feedback circuit 260 will trigger when the second DC voltage220 is above a threshold level which is presently preferred to be acombination of the voltage drop across the light emitting diode 300 ofoptocoupler 280 (preferably one volt) and the reverse break down voltageof zener diode 290. Upon triggering feedback circuit 260 will activatephototransistor 310 of the optocoupler 280. The activation ofphototransistor 310 causes a current to flow into the feedback terminal320. The current input into feedback terminal 320 is utilized to disableregulation circuit 240. Disabling regulation circuit 240 preventsswitching current at the operating frequency from flowing to energystorage element input 250 and prevents power from being supplied to theload 230. When regulation circuit 240 is not conducting a current source330 is triggered within the regulation circuit 240. The current source330 allows a small current to flow through a bypass terminal 340 ofregulation circuit 240 to charge regulation circuit power supply bypasscapacitor 350. Regulation circuit power supply bypass capacitor 350 isused to supply power to operate regulation circuit 240 when it isconducting. In this way when the second DC voltage 220 is above thedesired threshold level virtually no power is supplied to the load and aminimum amount of power is being consumed by the DC to DC converter 200.

It is presently preferred that at the moment when second DC voltage 220reaches a level below the threshold level, phototransistor 310 willcease conducting. When the phototransistor 310 is not conducting, nocurrent flows into feedback terminal 320 and regulation circuit 240 isenabled. When the regulation circuit 240 is enabled a switching currentat the operating frequency is supplied to the energy storage elementinput 250.

It is presently preferred, that the threshold level is equal to theregulated value of the output voltage, e.g. the second DC voltage 220.Alternatively, the output current can also be regulated by utilizing acurrent threshold. Referring to FIG. 3, a power supply 400 comprises abridge rectifier 410 that rectifies an input AC mains voltage. Powersupply capacitors 420 charge with the rectified AC mains voltage tomaintain an input DC voltage 430. A presently preferred range for inputDC voltage 430 is approximately one hundred (100) to four hundred (400)volts to allow for operation based upon worldwide mains voltages whichrange between eighty five (85) and two hundred sixty five (265) volts.The presently preferred power supply 400 also includes harmonic filtercomponents 440 which in combination with capacitors 420 reduce theharmonic current injected back into the power grid. Transformer 450includes a primary winding 460 magnetically coupled to secondary winding470. The secondary winding 470 is coupled to a diode 480 that isdesigned to prevent current flow in the secondary winding 470 when theregulation circuit 240 is conducting (on-state). A capacitor 485 iscoupled to the diode in order to maintain a continuous voltage on a load490 which has a feedback circuit 260 coupled to it. A presentlypreferred feedback circuit 260 comprises an optocoupler 280 and zenerdiode 520. The output of optocoupler 280 is coupled to the feedbackterminal 320 of regulation circuit 240. The presently preferredregulation circuit 240 switches on and off at a duty cycle that isconstant at a given input DC voltage 430. A regulation circuit powersupply bypass capacitor 350 is coupled to and supplies power toregulation circuit 240 when the regulation circuit 240 is in theon-state.

Operation of the power supply 400 will now be described. An AC mainsvoltage is input into bridge rectifier 410 which provides a rectifiedsignal to power supply capacitors 420 that provide input DC voltage 430to primary winding 460. Regulation circuit 240, which preferablyoperates at a constant frequency and about constant duty cycle at agiven input DC voltage 430, allows current to flow through primarywinding 460 during its on state of each switching cycle and acts as opencircuit when in its off state. When current flows through primarywinding 460 transformer 450 is storing energy, when no current isflowing through primary winding 460 any energy stored in transformer 450is delivered to secondary winding 470. Secondary winding 470 providesthen provides the energy to capacitor 485. Capacitor 485 delivers powerto the load 490. The voltage across the load 490 will vary depending onthe amount of energy stored in the transformer 450 in each switchingcycle which is turn dependent on the length of time current is flowingthrough primary winding 460 in each switching cycle which is presentlypreferred to be constant at a given input DC voltage 430. The presentlypreferred regulation circuit 240 allows the voltage delivered to theload to be maintained at a constant level.

It is presently preferred that the sum of the voltage drop acrossoptocoupler 280 and the reverse break down voltage of zener diode 520 isapproximately equal to the desired threshold level. When the voltageacross the load 490 reaches the threshold level, current begins to flowthrough the optocoupler 280 and zener diode 520 that in turn is used todisable the regulation circuit 240. Whenever regulation circuit 240 isin the off-state the regulation circuit power supply bypass capacitor350 is charged to the operating supply voltage, which is presentlypreferred to be five point seven (5.7) volts by allowing a small currentto flow from bypass terminal 340 to the regulation circuit power supplybypass capacitor 350. Regulation circuit power supply bypass capacitor350 is used to supply power to operate regulation circuit 240 when it isin the on-state.

When the regulation circuit 240 is disabled, an open circuit conditionis created in primary winding 460 and transformer 450 does not storeenergy. The energy stored in the transformer 450 from the last cycle ofregulation circuit 240 is then delivered to secondary winding 470 whichin turn supplies power to the load 490. Once the remaining energy intransformer 450 is delivered to the load 490 the voltage of the load 490will decrease. When the voltage at the load 490 decreases below thethreshold level, current ceases to flow through optocoupler 280 andregulation circuit 240 resumes operation either instantaneously ornearly instantaneously.

The presently preferred regulation circuit 240 has a current limitfeature. The current limit turns off the regulation circuit 240, whenthe current flowing through the regulation circuit 240 rises above acurrent threshold level. In this way regulation circuit 240 can reactquickly to changes such as AC ripple that occur in the rectified ACmains voltage, and prevents the propagation of the voltage changes tothe load. The current limit increases the responsiveness of theregulation circuit to input voltage changes and delivers constant poweroutput independent for the AC mains input voltage.

Although the presently preferred power supplies of FIGS. 2 & 3 utilizecurrent mode regulation and a feedback circuit that includes anoptocoupler and zener diode, the present invention is not to beconstrued as to be limited to such a feedback method or circuit. Eithercurrent or voltage mode regulation may be utilized by the presentinvention without departing from the spirit and scope of the presentinvention so long as a signal indicative of the power supplied to theload is supplied to the feedback terminal 320 of the regulation circuit240. Additionally, although the presently preferred power supplies bothutilize an optocoupler and zener diode as part of feedback circuitsother feedback circuits may be utilized by the present invention withoutdeparting from the spirit and scope of the present invention.

Advantages associated with the power supplies depicted in FIGS. 2 and 3include a “digital” on and off for the power supply making theregulation of the power supply extremely fast. Further, unlike knownpulse width modulated regulated power supplies, no compensation of theregulation loop is required. Additionally, in known analog pulse widthmodulated control the bandwidth, which is usually one to two kilohertz,is less than its switching frequency. The bandwidth of the presentlypreferred regulation circuit 240 is capable of operating at itsswitching frequency. The presently preferred switching frequency isbetween forty (40) and fifty (50) kilohertz. Also, since there is nocompensation loop or bias winding the cost of the power supply isreduced below the cost of known pulse width modulation regulated powersupplies and 50/60 Hz transformers utilized in linear regulationsolutions

Referring to FIG. 4, a presently preferred low power supply 600 producesan output power preferably ranging between zero (0) and one (1) watt,but can also be used with higher power levels without departing from thescope and spirit of the present invention. Bridge rectifier 610 Usreceives the AC mains voltage. Power supply capacitors 615 take therectified voltage and then generate a DC voltage 620 that is supplied toprimary winding 630 of transformer 640 and is then supplied to secondarywinding 650. The secondary winding 650 provides power to capacitor 660that supplies power to load 670. Load 670 has a zener diode 680 coupledin parallel with it. A regulation circuit 240 is coupled in series withprimary winding 630, so that when regulation circuit 240 is conducting,on-state, current flows through primary winding 630 and when regulationcircuit 240 is not conducting, off-state, current does not flow throughprimary winding 630. In the on-state power is supplied to regulationcircuit 240 by regulation circuit power supply bypass capacitor 350.

Operation of the low power supply 600 of FIG. 4 will now be described.The AC mains voltage input into bridge rectifier 610 is rectified andthe rectified voltage is supplied to power supply capacitors 615 thatprovide DC voltage 620. The DC voltage 620 is then provided to primarywinding 630 that is in series with regulation circuit 240. Regulationcircuit 240 preferably operates at a peak current limited duty cycle ata constant frequency and delivers power to the primary winding 630. Atthe beginning of each cycle when regulation circuit 240 is in theon-state the current through it ramps up at a rate determined by theinductance of primary winding 630 and the input DC voltage 620. When thecurrent reaches the current limit regulation circuit 240 goes into theoff-state. When current flows through the primary winding 630 energy isstored by transformer 640 and when no current flows through primarywinding 630 energy is delivered to load 670. A constant power isdelivered by the secondary winding 640 to the zener diode 680 and theload 670. As long as the load 670 consumes less power than delivered bythe secondary winding 640 at the zener diode 680 reverse break downvoltage, part of the power is consumed by the zener diode 680 and theoutput voltage is regulated at the reverse break down voltage.

Referring to FIG. 5, a current limit 710 (I) is designed into theregulation circuit 240 for faster response. The current flowing throughprimary winding 460 or 630 will rise to the level of current limit 710and then cease to flow. A high input voltage current 720 rises at afirst rate, while a low input voltage current 730 rises at a secondrate. The second rate is lower than first rate, but both currents reachthe current threshold limit 710 (I) although at different times. Therate of rise of the current is a function of the inductance of primarywinding (L) and magnitude of the input voltage. The power supplied tothe load is proportional to the area under the curves of the currentmultiplied by the input voltage, which is constant. Since the primarywinding current is limited at the current limit 710 (I) the powersupplied to the load, can be expressed as in Equation 1 below:

Based upon Equation 1 the power supplied to the primary winding by highinput voltage current 720 and low input voltage current 730 will be thesame, assuming the same regulation circuit 240 is operating with thesame current threshold limit 710 and at the same frequency (.function.).This is true regardless of the rate of rise of the primary windingcurrent. This means that the power supplied to the load in the powersupply of FIG. 3 or FIG. 4 will be constant and independent of the DCinput voltage 430 or 620. This means that the power supplied to the loadis independent of the AC Mains voltage. Thus, a constant power isdelivered utilizing the presently preferred ted regulation circuit 240

The power supplied to the load is a function of the current limit 710(I), frequency of operation (.function.) and the inductance of theprimary winding (L). Since the inductance of the primary winding andcurrent limit are determined by the circuit designer in designing thepower supply, the designer can design in the power delivered to the loadeasily and effectively by utilizing the presently preferred regulationcircuit 240.

It should be noted that the above discussion assumes, as is presentlypreferred, that the inductance of the primary winding is chosen suchthat the all of the energy input into the transformer is delivered ineach cycle of operation. As a result, the presently preferred primarywinding current begins at zero at the start of each cycle of operation.However, the present invention will still deliver power if theinductance of the primary winding is chosen such that not all of theenergy input into the transformer is delivered in each cycle ofoperation and the primary winding current begins at a non-zero value atthe start of each cycle of operation.

Referring to FIG. 6, a presently preferred regulation circuit 240comprises a MOSFET 800 that is coupled between a drain terminal 804 anda source terminal 806. MOSFET 800 is switched on and off according to adrive signal 850 input into its gate by first and-gate 810. The input offirst and-gate 810 comprises an output of first latch 820, a bypassterminal voltage indicator signal 845 provided by undervoltagecomparator 860, and a thermal status signal 870 from thermal shut downcircuit 880. Maximum duty cycle signal 830 determines the maximum timethat MOSFET 800 can conduct in each cycle of operation.

Thermal shut down circuit 880 monitors the temperature of the primarywinding by monitoring the temperature of regulation circuit 240 andprovides the thermal status signal 870 as long as the temperature isbelow a threshold temperature. It is presently preferred that thethreshold temperature is 135 degrees Celsius.

The inputs to latch 820 include an or-gate output signal 900 andand-gate output signal 910. The and-gate output signal is provided whenno phototransistor 310 current is provided to feedback input 320.Feedback gate 920 provides output when enable signal 905 is received andclock signal 930 is provided by oscillator 840. Additionally, firstcurrent source 940 will pull enable signal 905 to a logic high statewhen the current present in the phototransistor 310 is less than thecurrent source 940 current. In operation when enable signal 905 is high,the clock signal 930 is transferred to latch 820 by the and-gate 920,thereby setting the latch 820 and enabling that cycle to go through andturn on the MOSFET 800. Conversely, when the enable signal 905 is low,it blocks the clock signal from setting the latch 820, and keeps theMOSFET 800 off during that cycle.

Or-gate output signal 900 is provided by or-gate 945 when the currentthreshold limit 710 is reached or during the time when maximum dutycycle signal 830 is in an off state. In operation or-gate output signal900 will be provided when the maximum duty cycle signal is off or whenthe current limit 710 is reached in order to turn off the MOSFET 800.

Current threshold limit monitoring is performed by current thresholdcomparator 950 that compares the voltage level across the MOSFET 800on-resistance, if that voltage is above the current threshold limitvoltage 960 the current limit signal is triggered and the MOSFET 800 isturned off and then will not begin conducting until the beginning of thenext on-time when no current limit signal is provided.

In this way the presently preferred regulator circuit 240 turns off theMOSFET 800 after the current on cycle when the phototransistor 310 pullsthe enable signal 905 low and creates a condition where there will be noadditional power supplied to the load. When the phototransistor 310current falls below the first current source 940 current, enable signal905 is high due to the operation of current source 940 and MOSFET 800will resume operation upon the beginning of the next on-period of themaximum duty cycle signal 830.

Bypass circuit 970, which includes current source 330, regulates thepower level of regulation circuit power supply bypass capacitor 350 at avoltage level which is presently preferred to be five point seven (5.7)volts. This is done by charging the regulation circuit power supplybypass capacitor 350 when the MOSFET 800 is not conducting. Undervoltagecircuit 860 prevent the MOSFET 800 from conducting again until thevoltage at bypass terminal 340 reaches the desired voltage level.

Referring to FIG. 7, maximum duty cycle signal 830 is provided to firstinverter 1000 the output of which is provided to a first terminal 1005of bypass latch 1010 and to bypass and-gate 1015. The output of bypasslatch 1010 is provided to bypass and-gate 1015. The second inputterminal 1020 of bypass latch 1010 receives the output of second bypassinverter 1025 that receives input from bypass comparator 1030. Bypasscomparator 1030 determines whether the voltage at bypass terminal 340has reached the voltage level for terminating input to the regulationcircuit power supply bypass capacitor 350. A bypass MOSFET 1035 conductsor interdicts depending on the output of bypass and-gate 1015. When thebypass MOSFET 1035 conducts current source 330 allows current to flowbypass terminal 340 and allows the regulation circuit power supplybypass capacitor 350 to charge.

In operation bypass latch 1010 is turned on when the maximum duty cyclesignal 830 is high and MOSFET 800 is conducting. However, the output ofbypass latch 1010 is blocked by bypass and-gate 1015 from turning on thecurrent source 330 during this time. When the maximum duty cycle signal830 goes low MOSFET 800 turns off and the bypass and-gate 1015 will nolonger block the output of bypass latch 1010 from turning on currentsource 330. When the current source 330 is turned on, it charges theregulation circuit power supply bypass capacitor 350. When the bypassterminal voltage 1037 at the bypass terminal 340 reaches the voltagethreshold level, which is presently preferred to be five point seven(5.7) volts, the bypass latch 1010 is reset by the output of bypasscomparator 1030 and current source 330 is turned off. In this way bypassMOSFET 1035 will conduct only when the maximum duty cycle signal 830 islow and regulation circuit power supply capacitor 350 will charge onlywhen the MOSFET 800 is not conducting.

Referring to FIG. 8, the bypass terminal voltage 1037 will decreasewhile the maximum duty cycle signal 830 is high. When the maximum dutycycle signal 830 goes low, the current source 330 is activated and theregulation circuit power supply bypass capacitor 350 is charged which inturn increases the bypass terminal voltage 1037. It is presentlypreferred, that the bypass terminal voltage 1037 reaches the voltagethreshold prior to when the maximum duty cycle signal 830 goes high.Once the voltage threshold is reached, the bypass latch 1010 is resetand the bypass MOSFET 1035 ceases to conduct the regulation circuitpower supply bypass capacitor 350 will discharge until the next lowperiod of maximum duty cycle signal 830.

Referring to FIG. 9, normal oscillator current source 1040 provides acurrent to oscillator 840 when the oscillator is outputting the maximumduty cycle signal 830. A presently preferred speed-up current source1045 provides a current that has a greater magnitude than the currentprovided by the oscillator current source 1040. When both the speed-upcurrent source 1045 and the oscillator current source 1040 providecurrent to drive oscillator 840 the clock frequency is increased.Speed-up latch 1050 is set at the beginning of each clock cycle. Whenspeed-up latch 1050 is set, the speed-up switch 1055 is allowed toconduct allowing current from speed-up current source 1045 to increasethe clock frequency of oscillator 840. A speed-up or-gate 1060 willreset speed-up latch 1050 when the maximum duty cycle signal 830 is lowor when the latch 820 is set. It should be noted that latch 820 is setwhen the enable signal 905 is high. Therefore, in those cycles whenenable signal 905 is high and MOSFET 800 conducts the speed-up latch1010 is reset immediately at the beginning of that cycle and the clockfrequency of the oscillator is normal with only oscillator currentsource 1040 providing current. In those cycles when enable signal 905 islow and MOSFET 800 is not conducting, the speed-up latch 1010 is notreset until maximum duty cycle signal 830 is low and the oscillator 840operates at the predetermined higher frequency by the addition of thespeed-up current source 1045.

Referring to FIG. 10, oscillator 840 includes a saw tooth output 1070that provides a saw tooth waveform 1075. The saw tooth waveform 1075 isthe voltage across saw tooth capacitor 1080 that charges and dischargeswithin each cycle. A disable nor-gate 1085 is provided with the enablesignal 905 and the maximum duty cycle signal 830. Disable nor-gate 1085will provide an output when the enable signal 905 is low and the dutycycle signal 830 is low. The output of disable nor-gate 1085 is providedto clamp switch 1090 allowing the clamp switch 1090 to conduct and clampthe saw tooth waveform. 1075 to a voltage level between its high and lowpeaks during its falling edge. The presently preferred clamp switch 1090is a MOSFET. When the clamp switch 1090 conducts a current flows fromthe bypass terminal 340 through biasing transistor 1100 and clamp switch1090 to clamp the voltage level of saw tooth capacitor 1080. The voltageacross saw tooth capacitor 1080 is then clamped to a fixed value and theoscillator 840 ceases to function. In this way, oscillator 840 ceases tofunction when the load voltage is above the threshold level and theregulation circuit 240 is disabled.

Referring to FIG. 11, saw tooth waveform 1075 oscillates between ahigher voltage level and a lower voltage level. The presently preferredhigher voltage level is two (2) volts and the presently preferred lowervoltage level is one (1) volt. Once the enable signal 905 is removed,and the saw tooth waveform 1075 reaches the clamp voltage of saw toothcapacitor 1080 the saw tooth waveform 1075 is maintained at that clampvoltage. Once the enable signal 905 is provided again, the clamp switch1090 no longer conducts and the saw tooth waveform 1075 continues itscycle. Once the saw toothed waveform 1075 reaches the lower voltagelevel at resume time 1125 another clock cycle begins and oscillator 840resumes operation.

It is presently preferred that regulation circuit 240 comprises amonolithic device.

Referring to FIG. 12, maximum duty cycle signal 830 has an on-time 1200and off-time 1210. Enable signal 905 is provided and then stops at time1220 when feedback from phototransistor 310 is received. When the enablesignal 905 is terminated, drive signal 850 is maintained on for theremainder of on-time 1200. Once the on time 1200 is completed drivesignal is disabled. At time 1230, which is during on-time 1200 enablesignal 905 is provided again because feedback from phototransistor 310is no longer received. The drive signal 850 will not be provided againuntil the beginning of the next on-time 1200 of the maximum duty cyclesignal 830.

Referring to FIG. 13, in the alternate approach depicted in FIG. 9,maximum duty cycle signal 830 includes on-time 1200 and off-time 1210while the enable signal 905 is received. Also, during the on-time 1200drive signal 850 is provided. When the enable signal 905 is terminated,drive signal 850 is maintained on for the remainder of on-time 1200.Once the enable signal 905 is discontinued, it is presently preferredthat the oscillator 840 speeds up to a higher frequency by having ashortened on-time 1240 while maintaining the same length off-time 1210.When the enable signal 905 is provided again, MOSFET 800 resumesoperation in approximately half the time compared to the embodiment ofFIG. 12. The drive signal 850 will not be provided again until thebeginning of the next on-time 1200 of the maximum duty cycle signal 830.This approach may have some advantages in certain applications as itminimizes the response time of the regulation circuit 240. The shorterresponse time decreases of the voltage ripple at the load.

Referring to FIG. 14, in the approach depicted in FIG. 10, maximum dutycycle signal 830 includes on-time 1200 and off-time 1210 while theenable signal 905 is received. Also, during the on-time 1200 drivesignal 850 is provided. When the enable signal 905 is terminated, drivesignal 850 is maintained on for the remainder of on-time 1200. Once theenable signal 905 is discontinued, oscillator 840 ceases functioning.The drive signal 850 will not be provided again until the beginning ofthe next on-time 1200 of the maximum duty cycle signal 830, which willbe immediately upon receiving the enable signal 905. Like the embodimentof FIG. 13 this approach has the advantage of minimizing the responsetime of the regulation circuit 240 The shorter response time decreasesof the voltage ripple at the load.

While the embodiments, applications and advantages of the presentinvention have been depicted and described, there are many moreembodiments, applications and advantages possible without deviating fromthe spirit of the inventive concepts described herein. Thus, theinventions are not to be restricted to the preferred embodiments,specification or drawings. The protection to be afforded this patentshould therefore only be restricted in accordance with the spirit andintended scope of the following claims.

1. A regulation circuit for use in a DC to DC converter, comprising: afeedback circuit to be coupled an energy transfer element output toreceive a feedback signal to indicate whether the energy transferelement output is above or below a threshold level; and a controlcircuit to be coupled to a switch coupled to an energy transfer elementinput, the control circuit further coupled to the feedback circuit togenerate a drive signal to control the switch to regulate the energytransfer element output to approximately the threshold level, thecontrol circuit to maintain an on-time of a current cycle of the drivesignal and not allow an on-time of a next cycle of the drive signal inresponse to the energy transfer element output rising above thethreshold level.
 2. The regulation circuit of claim 1 further comprisingan oscillator coupled to provide an oscillating signal coupled tocontrol circuit, the oscillating signal to oscillate at a firstfrequency under a first set of conditions and at a second frequencyunder a second set of conditions, the control circuit to generate thedrive signal in response to the feedback signal and the oscillatingsignal.
 3. The regulation circuit of claim 2 wherein the oscillatingsignal does not oscillate under a third set of conditions.
 4. Theregulation circuit of claim 2 wherein one of the first and secondfrequencies is substantially zero.
 5. The regulation circuit of claim 2wherein the oscillator includes first and second current sources, thefirst current source to drive the oscillating signal under the first setof conditions and the first and second current sources to drive theoscillating signal under the second set of conditions.
 6. The regulationcircuit of claim 1 further comprising a current source coupled between abypass terminal and the first terminal, wherein the bypass terminal isto be coupled to a capacitor.
 7. The circuit of claim 1 wherein thecontrol circuit includes a latch coupled to be set in response to thefeedback signal and coupled to be reset in response to a duty cyclesignal, wherein the drive signal is coupled to be generated in responseto the latch.